In providing an anti-back-siphoning apparatus between a municipal water supply outlet and a solution mixing and dispensing device as described in the Hechler U.S. Pat. No. 3,862,640, a manual flow control valve is provided between a plurality of chambers in tandem, one housing anti-backflow valves and vents and the other a proportioning-mixing device. The valve manually cuts off water supply and blackfow of mixture while the vent valves automatically vent to atmospheric pressure when the water pressure drops to a critical low positive gauge pressure. Furthermore, since the mixture discharge orifice is continuously open to atmosphere, the venting should not only stop backflow contamination, but close backflow check valves at the water and concentrate inlets to the mixing device to prevent flow in either direction independently of any applied negative gauge pressure that remains on the concentrate that may be induced by gravity. Mixture still in the device can backflow and dilute the concentrate under the slight negative gauge pressure induced by gravity. Accordingly, means are desired to also prevent possible dilution and occasional loss of prime of the concentrate. Predominately, however, there are many occasions when it is repeatedly desirable to immediately alter operation and dispense pure water selectively as a wash between applications of the solution, or, apply a solution after a wash.
Furthermore, different chemicals may be dispensed through the device which are not compatible with the valve seals previously used and these should be capable of being readily flushed and changed.